i found this in this file: http://www.boost.org/doc/libs/1_43_0/boost/spirit/home/phoenix/core/actor.hpp
What does this syntax means?
struct actor ... {
...
template <typename T0, typename T1>
开发者_高级运维 typename result<actor(T0&,T1&)>::type // this line
I know what typename and templates are, my question is about actor(T0&,T1&)
syntax
thank you
The full declaration from that file reads:
template <typename T0, typename T1>
typename result<actor(T0&,T1&)>::type
operator()(T0& _0, T1& _1) const
{
/* snip */
}
If you break it down into chunks, it's easier to read:
template <typename T0, typename T1>
It's a template...
operator()(...) const
...a templated function-call operator function...
operator()(T0& _0, T1& _1) const
...which takes two arguments, by reference, of type T0
and T1
...
typename result<...>::type
...and returns a result whose type is the type
member of result
...
typename result<actor(T0&,T1&)>::type
...where the type of result
is paramaterised by the expression actor(T0&,T1&)
.
So this means that there is a template called result
and within result is a type called type
.
template <class T>
class result
{
public:
typedef ... type;
};
So that line is using that type from the template.
Because the compiler does not know what result<actor(T0&,T1&)>::type
is, you need to use typename
to tell the compiler to treat it as a type.
Update
actor(T0&,T1&)
is a function taking a T0&
and a T1&
and returning an actor
by value.
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